An active matrix substrate used in a liquid crystal display device, or the like, has a switching element, such as a thin film transistor (hereinafter “TFT”), for each pixel. An active matrix substrate having TFTs as switching elements is called a “TFT substrate”.
TFTs that have been used in the art include those using an amorphous silicon film as the active layer (hereinafter “amorphous silicon TFTs”), and those using a polycrystalline silicon film as the active layer (hereinafter “polycrystalline silicon TFTs”). It has been proposed to use an oxide semiconductor, instead of amorphous silicon or polycrystalline silicon, as the material of the active layer of a TFT, (e.g., Patent Document No. 1). Such a TFT is herein referred to as an “oxide semiconductor TFT”.
An oxide semiconductor has a higher mobility than amorphous silicon. Therefore, an oxide semiconductor TFT can operate faster than an amorphous silicon TFT. An oxide semiconductor film also has an advantage that it can be formed by a simpler process than a polycrystalline silicon film.
Liquid crystal display devices having oxide semiconductor TFTs have currently been developed. Since an oxide semiconductor TFT has a small size, it is possible to improve the pixel aperture ratio by using this as compared with cases where conventional TFTs are used. Thus, it is possible to realize a brighter display. Or the power consumption can be decreased by reducing the brightness of the backlight.
Using an oxide semiconductor TFT also makes it easier to provide a large storage capacitor Ccs. The storage capacitor Ccs is connected to the drain of the TFT and the liquid crystal capacitor Clc, and is used for retaining the voltage during the TFT OFF period. Even if a large storage capacitor Ccs is provided, it is possible to perform the write operation with generally the same frame frequency as those of conventional techniques because an oxide semiconductor has a high mobility.
In order to obtain a larger storage capacitor Ccs, a configuration is known in the art in which a storage capacitor Ccs is formed by using a pixel electrode, and a transparent conductive layer disposed underneath with an insulating layer interposed therebetween (e.g., Patent Document No. 2). In a configuration where the transparent conductive layer is provided under the pixel electrode, as described above, it is possible to obtain a large storage capacitor Ccs because it is possible to realize a relatively large overlapping area between the pixel electrode and the transparent conductive layer.